The determination of the rheological properties of rapidly sedimenting suspensions is very important in basic research and in many different fields. Examples of materials that are produced, processed or utilized as unstable particulate suspensions include mineral suspensions, coal-water slurries, magnetite dense media, flotation products, drilling muds, foods, agricultural chemicals, sewage, river deposits, and the like. The ease of transportation of such materials and their application to a particular purpose are dependent on the flow and viscous characteristics of the suspension. It is thus essential that the rheological properties of the materials can be accurately determined. This is not an easy task when dealing with materials where the particles are coarse or have a high density, or where the medium viscosity is low. In such materials, rapid sedimentation can occur thereby making it difficult to accurately measure the rheological properties of the materials.
In the mining and mineral processing industry, coal and many minerals are upgraded by means of a dense medium separation process, whereby valuable mineral components are separated from their gangue by virtue of their density difference. The dense medium used is generally a suspension of particles, usually magnetite or ferrosilicon in water, which experiences rapid sedimentation. The efficiency of separation depends on the viscous properties of this suspension, and hence optimization of the process is dependent on being able to accurately determine the rheological properties of the dense medium suspension.
Rotational viscometers have been used for many years to measure the rheological properties of liquids and suspensions. Samples of these materials are contained in the annular gap between coaxial cylinders, where either the cup or bob rotates depending on the system. The resistance to the rotation by the sample is a measure of its viscosity. For increased sensitivity, a double gap concentric cylinder arrangement is used. In both cases, there is just a small space between the bottom of the bob and the cup and between the top of the bob and the top of the cup.